1. Field of the Invention
The present invention relates to a droplet ejecting device and, more particularly, to a droplet ejecting nozzle.
2. Description of Related Art
Conventionally, various droplet ejecting devices such as ink jet printers form desired characters and figures on a printing sheet according to a predetermined signal. An ink droplet ejecting nozzle portion for ejecting ink droplets is the most important part of such droplet ejecting devices with respect to the printing quality of characters and figures formed on the printing sheet.
Water base dye ink, water base pigment ink, solvent pigment ink or hot melt ink can be used as ink for the above-described ink droplet ejecting devices. The ink droplet ejecting nozzles are necessarily designed in accordance with materials and shapes that are appropriate to the properties of ink to be used. These properties include surface tension and viscosity. It is especially important to control the wettability of the ink droplet ejecting nozzle portion for various inks. The wettability is determined by the physical property values such as the surface tension of the ink and the physical property values such as the surface tension of the material of the ink droplet ejecting nozzles.
Conventionally, nozzle plates have been made as follows to control wettability. The plate is typically made of a material whose wettability to the ink to be used is good (i.e. a small contact angle). A liquid-repellent process is made on the surface of the plate to form a liquid-repellent layer, and the desired number of ink droplet ejecting nozzles are formed in the plate. The nozzle plate made by the above-method has a different wettability to ink between the surface of the nozzle plate and the inner surface of ink droplet ejecting nozzles. Therefore, the nozzle plate meets wettability conditions such as smooth flow of ink in the nozzle holes and an ink-repellent property of the surface of the nozzle plate, which increases the printing quality and provides stable ink droplet ejecting.
However, when the desired number of ink droplet ejecting nozzles are formed in the nozzle plate having a liquid-repellent layer by the methods such as exima laser processing, microdrill processing, electric discharge machining and etching processing, the processing of the plate and the liquid-repellent layer is significantly different since each physical property of the nozzle plate material and the liquid-repellent layer differ. That is, in the ink droplet ejecting nozzle portion made by the above-mentioned method, burrs are easily made on the edge of the nozzle, and the liquid-repellent layer formed on the surface of the nozzle plate is easily damaged. Therefore, the printing quality deteriorates, and the stable ink droplet ejecting diminishes over time since the droplets are not ejected to a proper place.
Another method for making a nozzle plate is described as follows. After the desired number of nozzle holes are formed in the nozzle plate, the liquid-repellent processing is made on the surface of the nozzle plate to form the liquid-repellent layer. However, it is extremely difficult to prevent the adherence of the liquid-repellent material to the inner surface of the ink droplet ejecting nozzle regardless of a wet or dry liquid-repellent processing method. In some cases, the liquid-repellent material clogs the ink droplet ejecting nozzle holes.
Moreover, if a cleaning operation is executed, such as disclosed in U.S. Pat. No. 5,202,702, the liquid-repellent layer of the nozzle plate peels off by the sliding operation of the cleaning member with the surface of the nozzle plate. As a result, there arises a problem that ink spreads around the nozzle holes and is not ejected properly. Especially when pigment ink is used, the liquid-repellent layer is worn off due to the physical contact of the cleaning member and the nozzle plate and the abrasion phenomenon of the pigment, which is a solid included in the pigment ink. As a result, the liquid-repellent layer of the nozzle plate easily flakes off.